In certain electrical circuits, it is desirable to have a high repetition rate spark gap to operate as a switch for high currents flowing in the electric circuit. Most spark gaps have the common features of a housing defining a vacuum chamber in which is mounted a post-like central anode electrode, separated across a gap and electrically insulated from a concentric cylindrical cathode. Concentrically outside these two is a cylindrical housing to which a base is attached to form a chamber. The anode is mounted in the base. An electrically neutral seed gas, also sometimes referred to as a fuel gas, fills the gap between the anode and cathode. This electrically neutral seed gas is ionized during operation, thereby permitting an electric current to flow from the anode through the ionized gas to the chamber wall. This spark gap "switch" is returned to its "open" position either by permitting the ionized seed gas to recombine back into its initial electrically neutral state, or by removing the seed gas left in the chamber and introducing a new charge of gas. It is desirable for spark gaps to have a high repetition rate (rep rate) so they can be fired many times per second, on the order of 10.sup.4 times per second.
Two general categories of spark gaps now in use are the high pressure spark gap (HPSG) operating at pressures in the range of one (1) to one hundred (100) psi, and the low pressure spark gap (LPSG) operating at vacuum pressures on the order of 100 microns or less. These spark gaps are "conventional" in the sense that they use electrons as the ionizing atomic particle species to ionize the seed gas placed in the spark gap.
In the conventional high pressure spark gaps presently in use, an electron trigger source is used to ionize the neutral seed gas. As shown in FIG. 3 and discussed in more detail below, electrons efficiently ionize gas molecules when the electric energy of the electrons are equivalent to approximately 100 volts. By definition, the high pressure spark gaps contain a densely compressed electrically neutral seed gas with densities on the order of 10.sup.20 atomic particles per cubic centimeter. Therefore, there are many neutral seed gas molecules with which the electrons can collide. The electrons in fact do undergo many collisions with the seed gas molecules, and lose all their energy with each collision. It takes only one collision between an electron and a seed gas molecule for the electron to lose all its energy to the molecule. Hence, even if many hundreds of kilovolts are applied in creating the electric potential across the anode and cathode, the electrons never completely exceed the 100-volt energy associated with optimum ionizations.
These rapid and frequent electron-seed gas molecule collisions cause the seed gas to ionize very quickly, on the order of one (1) nanosecond. This is favorable from the standpoint that the spark gap "closes" very quickly; that is, the seed gas ionizes quickly to be capable of conducting an electric current from the anode to the cathode. However, the same physical collisional processes which provide a favorable ionization rate serve as a detriment to "opening" the switch; that is, the high density of the seed gas and the frequent electron-seed gas molecule collisions make it difficult for the ionized seed gas to recombine into its initial electrically neutral state. Therefore, to "open" the HPSG switch, it has been the practice to remove the high pressure ionized gas by connecting the chamber to vacuum pumps. After these pumps remove the high pressure ionized gas, a fresh charge of electrically neutral seed gas is then injected into the chamber for re-ionization.
Typical pressures in the high pressure spark gap are in the range of one (1) to one hundred (100) psi. Such pressures have the disadvantage of placing a high pumping requirement on the vacuum pumping system, thus requiring cumbersome pumping installations with pumps having capacities in the range of greater than 3000 cubic feet per minute (cfm) at 150 psig. An additional disadvantage is that the heavy pumping requirement severely limits the repetition rate of the switch; it can only fire at an upper rate of 10.sup.3 times per second.
Many pulsed power devices make use of spark gap switches to suddenly close the electrical circuit of transmission lines charged by voltage. For example, the Experimental Test Accelerator (ETA) electron beam accelerator at the Lawrence Livermore National Laboratory uses Blumlein transmission lines at about 5 ohms characteristic impedance charged to 250 kV. The switch current is 50 kA, and 25 kA 50 ns current pulses are delivered to the electromagnetic induction accelerating units. High gas pressure triggered spark gap switches are used in the ETA. The seed gas is nitrogen with the addition of 8% SF.sub.6 ; the seed gas pressure is approximately 8 atmospheres.
Conventional low pressure spark gaps (LPSG) have the inverse problem. As its name implies, the low pressure spark gap has a low seed gas pressure, typically in the range of several tens to several hundred microns. The LPSG therefore has a low gas density, typically five orders of magnitude (i.e., 10.sup.5) less than the pressure found in the high pressure spark gap. Because there is low pressure, there is also a low density of seed gas molecules. Thus the electrons traveling between the anode and cathode undergo relatively few collisions with the seed gas molecules; the electrons are accelerated up to high kinetic energies due to the voltage between the electrodes.
As the electrons "run away" in their acceleration to high energy levels (on the order of several tens of kilovolts), their ability to ionize the seed gas drops sharply, resulting in a seriously degraded ionization rate. As a result, the low pressure spark gap switch closes poorly because of the low population density of ionized seed gas. This has the unfortunate consequence of creating a slow current rise (on the order of several tens of nanoseconds). However, the positive aspect of the degraded ionization rate is that the rapid electron mobility allows for very quick recombination of the ionized gas back into an electrically neutral gas. Thus the low pressure spark gap has quick recovery time, meaning that the switch re-opens quickly upon removal of the energy which ionizes the seed gas. There is no requirement for extensive pumping systems as is found in the high pressure spark gap, and there is no close limit on the repetition rate. "Close limit" as used here is defined as the time it takes the seed gas to recombine and the switch to recover to be ready for another firing of rapidly pulsed current. It is desirable to have a recombination time (i.e., close limit) of fractions of a microsecond, thereby permitting a rep rate in the megahertz range.
The recovery of the voltage-holding ability of both the HPSG and LPSG switches following discharge is hastened by blowing the seed gas through the electrode space at a velocity of about 4 cm/millisecond. Under these conditions, the switches have a maximum repetition rate of about 1 kHz. For some applications of these ETA accelerators, a faster repetition rate is desired. A switch operating near the low pd branch (pd branch as used here is defined as the gas pressure (p) times electrode spacing distance (d) as a function of the voltage holding capability) of the Paschen self-breakdown curve is expected to have faster recovery. This is because the ions and electrons resulting from a particular discharge have a mean free path through the seed gas comparable with electrode spacing, and so the ionized particles should rapidly recombine at the surfaces of the electrodes. For example, to be acceptable for ETA purposes, the triggered switch should have a fast rise time of current (on the order of 5 ns) and low jitter (having a width of the distribution of firing time delays on the order of a few ns).
This ionization rate limit is a fundamental physics limit. Ionization rate (the buildup of the density n.sub.p of the seed gas after it has been ionized in the gap between the cathode and the anode) is dependent on the current density J of the ionizing particles and their mean free path lambda (.lambda.) for an ionization event to occur. The physics relationships are expressed in Equation 1 as follows: ##EQU1##
In Equation 1, the subscripts refer to the type of particle which ionizes the electrically neutral seed gas: e=electrons, i=protons (positive ions), and o=neutrals (neutral ions). The limitation on the rate (measured in density per second) at which ionization of the seed gas can occur is established by the functional energy dependence of lambda, defined as the mean free path for an ionization event to occur. Customary notation is to define mean free path lambda as equal to 1/n.sub.g times sigma (.sigma.) (E), where n.sub.g =seed gas density of the seed gas that is to be ionized, and sigma times (E)=the energy-dependent "cross section"=the probability of ionization occurring in response to the incident ion species (i.e., positive, neutral or electrons) comprising the current density J (measured in units of amps per cm.sup.2 of seed gas).
For the electron-triggered high voltage LPSG, only the first term (J.sub.e /lambda.sub.e) is operable because it is the electron current flowing between the anode and cathode which establishes the rate of ionization of the electrically neutral seed gas. As illustrated by FIG. 3, the rate of ionization is inherently limited due to the optimum value for the energy-dependent cross section (sigma times E), which for high voltage is limited to an upper limit of approximately 120 kV. Ionization rate could be greatly enhanced if the second and third terms of Equation 1 on the right hand side of the equal sign could be brought into operation; however, up until now it has not been possible to do this.
To summarize, the desirable features of a low pressure spark gap (LPSG) when compared to the high pressure spark gap (HPSG) are (1) the inherent rapid recovery of the LPSG due to fast recombination of the ionized seed gas into its electrically neutral molecular configuration, and (2) the obvious mechanical system advantage of the LPSG by greatly reduced gas pumping requirements when contrasted with the high pressure spark gap. On the other hand, the major limitations of the low pressure spark gap when compared to the high pressure spark gap are (1) the LPSG's relatively long current rise time (on the order of 10's of nanoseconds) at high voltage (around 100 kV), and (2) anode damage in the LPSG due to electron bombardment early in the discharge when the potential difference in the gap between the anode and cathode is still high (on the order of 100 kV). Both of these limitations result from the conventional electron-ionized low pressure spark gap having the characteristic of being ionization-rate limited for high voltage, typically in the range of greater than 100 volts. For the purposes of spark gaps, high voltage is considered any voltage in the range of 120 kV and above.